1. Technical Field
The present invention relates to the field of electronic power conversion and, more particularly, to multiphase medium-to-large power DC power supplies having electronic power factor correction for each phase.
2. Discussion of Relevant Art
In medium power level applications, mainly in 1-10 kW range, two phase 208/120 V electrical service is frequently used along with single phase 208 V or 240 V services. There are no conventional converters which will operate from either two phase or single phase service while providing unity power factor loading on the available phases.
There are numerous single phase AC-DC converters with electronic power factor correction. For example, U.S. Pat. No. 5,019,952 to Smolensky et al. discloses a conventional converter having power factor correction. The converter includes a rectifier bridge for rectifying a bipolar AC voltage waveform into a unipolar voltage which is fed to an electronically controlled DC-DC boost converter. The electronic controller pulsewidth modulates a switching semiconductor of a boost converter circuit so that current drawn from a power line via the rectifier bridge is proportional to the power line voltage waveform thus assuring a power factor close to unity.
One disadvantage of this circuit is that current flows through three diodes to the load, i.e., two diodes in the rectifier bridge and one in the boost converter, causing excessive power losses. Another disadvantage of the Smolensky et al. converter is that the modulation of the switching semiconductor causes large high frequency voltage changes on the negative terminal of the rectifier bridge, producing a substantial electromagnetic interference current in the power line conductors. Accordingly, costly filters are required between the rectifier bridge and the power line for the converter to comply with electromagnetic interference regulations. Yet another disadvantage of the Smolensky et al. converter is that the voltage applied to the switching semiconductors is large, equalling or exceeding of the maximum peak voltage of the AC waveform. Consequently, costs associated with switching semiconductors are high, particularly for MOSFETs which are commonly used in converter circuits. MOSFET costs increase approximately as a square of the drain-source voltage rating of a MOSFET increases.
U.S. Pat. No. 4,940,929 to Williams discloses a converter having an input filter for reducing the electromagnetic interference described above. Nevertheless, the Williams converter still has the disadvantages of the Smolensky et al. converter, i.e., excessive power losses and excessive filter and semiconductor costs.
U.S. Pat. No. 4,980,812 to Johnson, Jr. et al. discloses a circuit configuration having a boost converter between the power line and a rectifier bridge. The disclosed converter overcomes the disadvantagee of excessive power losses in rectifier diodes and excessive costs of input filters, but does not solve the disadvantage of high semiconductor costs.